Polarization maintaining optical fiber connector and method of tuning (PM connector)

ABSTRACT

An apparatus for tuning PM optical fiber connections has a first assembly including a light source which is connected to a first coupling stage having a rotatable polarizer, and a second assembly having a second coupling stage and a rotatable polarization analyzer for directing light transmitted through a terminated PM jumper cable coupled between the first and second coupling stages to a power meter. The output of the power meter is applied to a processing unit which, in turn, controls a rotation arrangement for the polarizing and analyzer. Crosstalk of this jumper is determined by ascertaining the angular positions of the maximum and minima outputs of the rotating analyzer, and the terminating connectors of the jumper are turned to the maximum value by aligning with a connector key. The process similarly applies to the connections between two jumper cables, with the alignment of the slow wave vectors thereof having achieved by determining the difference in the angular positions of the maximum output, and determining therefrom the positions of the reference keys of both connectors.

RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.09/811,074 filed Mar. 16, 2001 the disclosure of which is incorporatedherein by reference, and to U.S. patent application Ser. No. 10/151,613filed May 20, 2002 and U.S. Pat. No. 6,619,856 issued Sep. 16, 2003,filed concurrently herewith.

FIELD OF THE INVENTION

This invention relates to connectorizing and tuning polarizationmaintaining (PM) optical fibers.

BACKGROUND OF THE INVENTION

In optical fiber communications, connectors for joining fiber segmentsat their ends, or for connecting optical fiber cables to active orpassive devices, are an essential component of virtually any opticalfiber system. The connector or connectors, in joining fiber ends, forexample, has, as its primary function, the maintenance of the ends in abutting relationship such that the core of one of the fibers is axiallyaligned with the core of the other fiber so as to maximize lighttransmissions from one fiber to the other, or, put another way, toreduce insertion loss. Another goal is to minimize back reflections.Alignment of these small diameter fibers is extremely difficult toachieve, which is understandable when it is recognized that the modefield diameter MFR of, for example, a singlemode fiber is approximatelynine (9) microns (0.009 mm). The MFR is slightly larger than the corediameter. Good alignment (low insertion loss) of the fiber ends is afunction of the transverse offset, angular alignment, the width of thegap (if any) between the fiber ends, and the surface condition of thefiber ends, all of which, in turn, are inherent in the particularconnector design. The connector must also provide stability and junctionprotection and thus it must minimize thermal and mechanical movementeffects.

In the present day state of the art, there are numerous, different,connector designs in use for achieving low insertion loss and stability.In most of these designs, a pair of ferrules (one in each connector),each containing an optical fiber end, are butted together end to end andlight travels across the junction. Zero insertion loss requires that thefibers in the ferrules be exactly aligned, a condition that, given thenecessity of manufacturing tolerances and cost considerations, isvirtually impossible to achieve, except by fortuitous accident. As aconsequence, most connectors are designed to achieve a useful,preferably predictable, degree of alignment, some misalignment beingacceptable.

However, in connecting or terminating polarization maintaining (PM)fibers, such is not the case. Many optical fiber components, such as,for example, interferometers and sensors, lasers, and electro-opticmodulators, are extremely sensitive to and dependent upon, for properoperation, the polarization of the light. Even very slight alterationsor changes in the light polarization orientation can result in wideswings in the accuracy of response of such devices. PM fiber haspolarization-dependent refractive indices, and the speed of light in anoptical fiber is inversely proportional to the magnitude of therefractive index. A birefringent optical fiber is one having twopolarizations having different velocities of propagation, thus givingrise to a “fast” wave and a “slow” wave. In a PM fiber, the polarizationof a linearly polarized light wave input to the fiber, with thedirection of polarization parallel to that of the one of the twoprincipal polarizations, will remain or be maintained in thatpolarization as it propagates along the fiber, hence the term“polarization maintaining.” If the polarization of the light wave is tobe maintained at a splice or other connection, the principal axes ofbirefringence of the two joined fibers must be aligned in parallel,otherwise there will be polarization cross-coupling, i.e., crosstalk,which is highly undesirable. Thus, where two PM fibers, for example, areto be connected together, they should be terminated carefully to reducethe crosstalk during the connectorization process. Also, the connectorsmust be capable of aligning then maintaining the fiber orientation tothe connector key position. Connectors with tolerances adequate forconnecting non-PM fibers usually are inadequate for maintainingpolarization alignment at the connector junction.

Typical PM connector requirements are an insertion loss of less than 0.3dB, and the prior art PM connector arrangements comprise numerous,different connector configurations aimed at meeting these requirementsfor different connectors, such as an SC type connector as shown in U.S.Pat. No. 5,216,733 of Ryo Nagase et al. The connector of that patentcomprises a ferrule body and a ring shaped flange having a keywaymounted on the periphery of the ferrule body. Alignment is achieved byrotating the ferrule body with respect to the flange keyway. Thecombination of ferrule and flange comprises a plug which is insertedinto a push-pull SC connector having a key therein for mating with theflange keyway and springs bias the flange in the longitudinal directionto maintain the alignment.

In U.S. Pat. No. 4,784,458 of Horowitz, a splice joint for PM fibers isshown wherein aligned fibers are joined with UV curing epoxy, and thejoint is overlaid with epoxy cement for rigidity. Such a joint ispermanent, and does not function as a connect-disconnect optical fiberconnector.

U.S. Pat. No. 5,561,726 of Yao discloses an apparatus for controllingthe polarization state of the light within a fiber by squeezing aportion of the fiber to produce a birefringent fiber, and the squeezeris then rotated to change the polarization of the light within thefiber. The device is not a connector, but is intended for use withpolarization sensitive devices such as interferometers and electro-opticmodulators, however, it may also be used with connectors for connectingtwo PM fibers.

It is common practice in the prior art for creating PM fibers to includea pair of rods in the fiber cladding which extend parallel to the coreas shown in U.S. Pat. No. 4,515,436 of Howard et al. Such rods, whichare preferably of glass, are, in manufacture of the fiber, included inthe fiber preform from which the fiber is drawn. As the fiber is drawn,the rods are accordingly diminished in diameter and are located withinthe cladding, preferably on either side of the core. The rods havedifferent thermal expansion characteristics than the surrounding glass,and the stress they exert on the core causes the index of refraction tochange along that axis. The axes then have different indices ofrefraction value and thus propagate light at different speeds.Variations on the two rod arrangement are also known, such as theelliptical stress member disclosed in U.S. Pat. No. 5,488,683 of Michalet al. Also, squeezing the fiber to create birefringence, as shown inthe aforementioned Yao patent is feasible. The two rod PM fiber, socalled “Panda” type PM fiber, however, has proven quite satisfactory inuse, and it is toward the connectorization of such a fiber that thepresent invention is directed, although other types of PM fibers may beused with the present invention.

SUMMARY OF THE INVENTION

In the copending U.S. patent application Ser. No. 10/151,613 of Lampert,et al. and U.S. Pat. No. 6,619,856 of Lampert, et al, are shown,respectively, a PM connector plug and an adapter therefor the principlesof which are applicable to any of a large number of optical fiberconnectors, but are embodied in a modified LC connector in thoseapplications. For optimum performance, i.e., maximum transmission of apolarized beam, it is highly desirable to provide accurate rotationalpositioning of better than ±1° or even as accurate a <¼° betweenconnectors equipped with polarization maintaining fibers.

The present invention is an apparatus and method for tuning the PMconnectors of those applications to achieve this desideratum.

When a PM jumper cable, for example, is terminated by connectors, it ismost desirable that the cable/connector combination be tuned to alignthe fiber slow axis with the connector key which serves as a referencepoint. In accordance with the present invention, there is provided atuning apparatus for performing the tuning method of the invention whichyields extremely accurate rotational positioning of the connectors.

The apparatus, which is similar to that shown in TIA/ETA StandardFOTP-193, comprises a first assembly including a coupling stagecomprising a light source, and a polarizer interposed between first andsecond connector adapters and connector plugs. A second assembly havinga second coupling stage, spaced from the first coupling stage comprisesa connector adapter (the second coupling stage), a power meter, theoutput of which is connected to a PC, and another polarizer (oranalyzer). Both polarizer and analyzer can be rotated to any angle andcontrolled by a rotation controller connected to the PC. In use, ajumper cable, for example, terminated by connector plugs, is insertedinto the adapters in the first and second coupling stages, and thepolarizer in the first stage is rotated to match the slow polarizationaxis of the connector, determined by the increased power reading. Linearpolarized light is then launched into the slow axis of the PM fiber. Theanalyzer in the second stage is rotated and the output power variesbetween maximum and minimum, as indicated by the power meter. As will bediscussed hereinafter, the crosstalk in dB is calculated as thedifference between maximum and minimum power.

The tuning of the PM connector is to set the PM fiber slow axis tocorrespond to the key position of the connector, which preferably is theconnector latch or latching arm. Therefore, two joined PM jumpers canhave the same slow axis alignment according to key position to minimizecrosstalk due to misalignment. Once the polarization direction of theanalyzer is aligned to the connector key which can be regarded as amaster position, the tuning process can easily be done by matching thefiber slow axis to the analyzer direction as indicated by output lightpower. In order to align the analyzer to the key or master position, apair of PM jumper cables are connected to each other and to the firstand second stages, and the crosstalk of the connection is then measured,and one connector is tuned for the lowest crosstalk in the connection oftwo jumpers.

One of the jumpers is then moved and the other is connected between thefirst and second stages. The angle of the maximum output is defined aszero degrees. By measuring the crosstalk, the analyzer can be aligned tothe slow axis of the fiber and the analyzer position is recorded. Theone jumper is then removed and the second is connected between thestages and the analyzer is also aligned to the second jumper's slow axiswhich generally will occur at a different angle than that of the onejumper.

The position of the key will be midway between the two angles and theanalyzer is rotated to the master position and thus the ferrules of theconnectors are rotated to this value, which is designated as zero. Atthis point the slow wave orientation of the connectors is parallel tothe connector key, and the connector of the PM jumpers are optimallylined. With the master analyzer position thus determined, subsequentalignment of connectors becomes a single rotation of the ferrules toconform.

With the connector plug and adapter of the aforementioned Lampert, et alapplications, the ferrules of the connector plug are maintained, withvery slight possibility of variation, in the optimum tuned position.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a prior art LC type connectorplug;

FIG. 2 is a perspective view of the plug of FIG. 1 as assembled;

FIG. 3a is a view of the flanged barrel member of the plug of FIG. 1;

FIG. 3b is an end view of the barrel member of FIG. 3;

FIG. 4a is an end view of the plug of FIGS. 1 and 2;

FIG. 4b is a cross-sectional view of a portion of the housing of theplug of FIG. 4A;

FIG. 5 is a cross-sectional view of the plug of the previous figures asassembled and terminating an optical fiber;

FIG. 6 is an exploded view of an alternative form of tunable barrelmember for a connector plug;

FIG. 7 is a side elevation view of the assembled barrel member of FIG.6;

FIGS. 8 and 9 are two views of the plug housing for use with PM opticalfiber;

FIG. 10 is a section along the line A—A of FIG. 8;

FIG. 11 is a cross-sectional elevation view of one embodiment of the PMplug terminating an optical fiber;

FIG. 12 is a perspective view of another embodiment of the PM plug;

FIG. 13 is a perspective view of a connector adapter for use with theplug of FIG. 5, for example;

FIG. 14 is a top plan view of the adapter of FIG. 13;

FIG. 15 is a side elevation view of the adapter of FIG. 13;

FIG. 16 is a front elevation view of the adapter of FIG. 13;

FIG. 17 is a diagrammatic representation of the relationship between twoPM ferrules to be connected together;

FIG. 18 is a diagram of the apparatus for tuning an optical fiberjumper;

FIG. 19 is a diagram of one of the steps in tuning the optical fiberjumper;

FIG. 20 is a diagram (or graph) of the variation in output power as thepolarizer and analyzer are rotated;

FIG. 21 is a graph showing the effect of misalignment on crosstalk;

FIG. 22 is the apparatus of FIG. 18 as set up to determine the masterreference position;

FIG. 23 is a graph resulting from the operation of the apparatus of FIG.23; and

FIGS. 24 through 27 are diagrams illustrating the several steps inestablishing the master reference position.

DETAILED DESCRIPTION

FIG. 1 is an exploded perspective view of the principal components of anLC type connector 11 as disclosed in the aforementioned U.S. patentapplications and U.S. Pat. No. 6,155,146. It is to be understood thatthe principles of the present invention are also applicable to othertypes of connectors, such as an ST, SC, or other amenable tomodification to incorporate these principles. Connector 11 comprises aplug housing formed of a front section 12 and a rear section 13 havingan extended portion 14 which fits into section 12 and latches thereto bymeans of slots 16-16 in front section 12 and latching members 17—17.Members 12 and 13 are preferably made of a suitable plastic material.Front section 12 has a resilient latching arm 18, having latchingshoulders 20, extending therefrom for latching the connector 11 in placein a receptacle or adapter. The arm 18 and shoulders 20 together definea latch. Rear section 13 has extending therefrom a resilient arm ortrigger 19, the distal end of which, when the two sections 12 and 13 areassembled, overlies the distal end of arm 18 to protect it from snaggingand to prevent nearby cables from becoming entangled. Usually latch arm18 and guard 19 are molded with their respective housing sections 12 and13, respectively, and form “living hinges” therewith, which enable themto be moved up and down between latching and unlatching positions. Frontsection 12 has a bore 21 extending therethrough which, when the partsare assembled, is axially aligned with a bore 22 extending through rearsection 13. The bores 21 and 22 accommodate a barrel assembly 23 whichcomprises a hollow tubular member 24 having a bore 25 extendingtherethrough and having a ferrule holding apparatus shown here as anenlarged flange or barrel member 26 from which extends a ferrule 27which may be made of a suitably hard material such as, preferably,ceramic, glass, filled-plastic, or metal. Ferrule 27 has a bore 28extending therethrough for receiving and holding an optical fibertherein. When the connector 11 is assembled, a coil spring 29 surroundsthe tubular portion 24 of the assembly 23, with one end bearing againstthe rear surface of flange 26 and the other end bearing against aninterior shoulder in rear section 13, as will best be seen in subsequentfigures.

In practice, the uncoated portion of the optical fiber is inserted intobore 28 of ferrule 27 and adhesively attached thereto. Spring 29 iscompressed as the sections 12 and 13 are connected and supplies aforward bias against the rear of flange 26 and, hence, to ferrule 27.This arrangement of ferrule 27 and spring 29 is considered to be a“floating” design. Prior to connection, the spring 29 causes ferrule 27to overtravel its ultimate connected position. When connector 11 isconnected within a suitable adapter and the distal end of ferrule 27butts against the corresponding ferrule end of another connector or ofother apparatus, spring 29 will be compressed, thereby allowing backwardmovement of ferrule 27 to where its end, and the end of the abuttingferrule, lie in the optical plane (transverse centerline) between thetwo connectors.

The rear end of rear section 13 has a ridged member 31 extendingtherefrom for attachment of optical fiber cable and a strain reliefboot, not shown. For protection of the distal end of ferrule 27 duringhandling and shipping, a protective plug 32, sized to fit within bore21, is provided. FIG. 2 depicts the assembled connector 11 in itsshipping or handling configuration.

As best seen in FIGS. 3a and 3 b, flange 26 has a hexagonally shapedportion 33 and a front tapered portion 34 with tuning notches 35, asshown in U.S. Pat. No. 6,155,146, which can be a tapered extension ofthe hexagonal portion. While the following discussion relates to amulti-faceted ferrule holding member, it is to be understood that theterm “faceted” is intended to include other locating arrangements suchas, for example, slots or splines, such as are shown in, for example,the U.S. Pat. No. 6,155,146 patent. As shown in FIGS. 4a and 4 b, frontsection 12 has a flange seating cavity 36 formed in a transverse wall 37thereof which has a hexagonally shaped portion 38 and a tapered portion39 dimensioned to receive and seat surface 34 of flange 26. That portion41 of bore 21 immediately to the rear of portion 38 has a diametersufficient to allow rotation of flange 26 when it is pushed to the rearagainst spring 29 and disengaged from the cavity 36. Thus, as will bediscussed more fully hereinafter, when flange 26 is pushed to the rearit may be rotated and, when released, re-seated by spring 29 withtapered portion 34 acting as a guide and centering arrangement. Thehexagonal configuration makes it possible to seat the flange 26 in anyof six angular rotational positions, each sixty degrees (60°) apart. Ithas been found that a flange having fewer than six sides cannot berotated in the assembled connector unless the diameter of bore portion41 is increased because the diagonal of a four sided flange is too greatfor rotation of the flange. However, increasing the diameter of portion41 seriously weakens the walls of the housing section 12. Further, inthe tuning of the connector it has been found that six sides gives amore accurate tuning for reduction in insertion loss. The use of aflange with more than six sides is possible, and gives an even greatertuning accuracy by creating smaller increments of rotation. However, theincreased accuracy is not sufficiently great to justify the increaseddifficulty in achieving a stable and firm seating of the flange. As thenumber of flange sides is increased, the periphery thereof approaches acircular configuration, which would possibly be undesirably rotatableeven when seated. As a consequence, it has been found that a six sidedflange is optimum for tuning non-PM type connector plugs. For the PMtype fiber connections, greater precision, including incremental controlof angular orientation of the polarized fiber in the ferrule is requiredif optimum light transmission with polarization unimpaired or altered isto be realized.

The present disclosure comprises three separate apparatuses and a tuningmethod for achieving optimum or near optimum polarization maintenanceand transmission through the connector assembly, which comprises aconnector plug and an adapter therefor.

PM Connector Plug

In the foregoing, the tuning process for non-PM connections is shown anddiscussed. The PM connector plug, which is basically a modified LC typeconnector plug, is shown in cross-section in FIG. 5 and comprises a plughousing 51 which includes a front section 52 and a rear section extendercap 53, within which is contained a barrel assembly 54 having a fiberbearing ferrule 56 mounted thereto. The barrel assembly 54 has anenlarged nut, such as a hexagonal nut 57 which is a light press fit on atubular member 58 through which the coated fiber 59 passes. Nut 57 has asloping front surface 61 and is held in a matching seat 62, which has asloped surface 60 for receiving surface 61, pressed into engagement bymeans of a coil spring 63, as shown. Seat 62 is also hexagonal so thatbarrel assembly 54 is prevented from rotating when seated in frontportion 52. A boot 64 extends from the rear extender cap 53 inaccordance with common practice. Front section 52 has a resilient latchcomprising latching arm 66 and latching shoulders 20 mounted incantilever fashion thereon, which is a feature of an LC connector plug.

FIG. 6 is an exploded perspective view and FIG. 7 side elevation view ofthe barrel assembly 54 comprising the tubular member 58 in which theferrule 56 is fixedly mounted. Member 58 has a bore 67 extendingtherethrough for receiving the coated fiber 59, and the front end 68 hasfirst and second tuning notches 69 and 71. Nut 57 is mounted in a lightpress fit on the front portion 68 of tubular member 58 and butts againsta stop ridge 72. By light press fit is meant a fit such that withapplication of a substantial rotational torque on tubular member 58 itcan be rotated with respect to nut 57, yet the fit is tight enough thatrelative rotation between the member 58 and nut 57 will not occur underthe forces, if any, likely to be encountered in use. Thus incrementalrotation of the fiber containing ferrule 56, which is fixed in tubularmember 58, relative to the nut 57, may be performed. As thus fardescribed, the connector plug 51 is substantially similar to theaforementioned Lampert et al. application. In keeping with the necessityof eliminating as much play or float as possible so that subsequentpolarization tuning may be maintained, connector plug 51, moreparticularly the plug housing, comprising front and rear sections 52 and53, is made to be a firm fit within the connector adapter, describedhereinafter, which is the subject of U.S. Pat. No. 6,619,856 which wasfiled concurrently herewith. In addition, as best seen in FIGS. 8, 9,and 10, the latching arm 66 has a cross-section that is in the form of atruncated wedge, with the sides 73 and 74 thereof being at an angle θ ofapproximately four to eight degrees (4°-8°) to the vertical, as shown inFIG. 10, although other angles or angle ranges might be used. Latchingshoulders 20 may be tapered, as shown in FIG. 10.

In FIG. 11 there is shown a second embodiment 76 of the connector plug.For simplicity, like parts to those in FIGS. 5-7 bear the same referencenumerals. FIG. 12 is an exploded perspective view of a portion of theplug 76, illustrating the unique details thereof.

As can be seen in FIG. 11, tubular member 54 is made with the nut 57integral therewith, although the arrangement of FIGS. 6 and 7 may alsobe used. The sloped surface 61 bears against the sloped surface 60within front section 52, as is the case with the embodiment of FIG. 5.However, front section 52 does not have, within the bore thereof, thehexagonal seating or locating surface 62 of the embodiment of FIG. 5.Instead of the surface 62, member 53 has extending longitudinallytherefrom toward the connector end of the plug 75 three separateresilient clamping arms 77, 78, and 79, the distal end of each of whichends in a clamping pad 81, as best seen in FIG. 12. Arms 77, 78, and 79are radially positioned 120° from each other and pads 81 have flat facesfor bearing against corresponding flat surfaces 33 on nut 57 thusforming a three-jaw collet. The diametric spacing of the pads 81 isslightly less than the corresponding faces of nut 57 against which theybear, thus insuring a positive clamping action on nut 57. However, theresilience of the arms, which are preferably made of a suitable plastic,is such that nut 57 may be rotated with respect thereto upon applicationof sufficient torque. Thus nut 57, and consequently ferrule 56, may berotated with respect to latching arm 66, and clamped firmly in placeafter such rotation. Further, as pointed out hereinbefore, where, as inthe embodiment of FIGS. 6 and 7, the nut 57 is not integral with tubularmember 54, incremental rotations of ferrule 56 with respect to latchingarm 66 are possible. A circular ridge 82 surrounds the member 53 andrides in a corresponding groove 83 in front section 52 to permitrelative rotation of the barrel assembly and three-jaw collet of FIG. 9with respect to latching arm 66, which likewise permits incrementalangular position changes of ferrule 56 with respect to latching arm 66.After the turning process, which will be discussed more fullyhereinafter, ridge 82 may be cemented within groove 83 to maintain thetuned position of ferrule 56 with respect to arm 66. While the colletchuck formed by the arms 77, 78, and 79 is shown with three arms, it isto be understood that fewer arms, or more arms, may be used so long asthe barrel assembly is located and firmly held in place within frontportion 52.

The tapered cross-section of arm 66 in both embodiments is intended tofit within as tapered slot within the adapter, to be discussed morefully hereinafter, but can fit within a straight side slots as wellinasmuch as the tapered sides of the arm 66 will engage the straightsided walls at some point, thus limiting lateral float. Thus the plugmay be used in a typical LC connection as well as a PM connection.

The PM connector plug as shown and described herein forms the basis ofU.S. patent application Ser. No. 10/151,613.

PM Adapter

FIG. 13 is a perspective view of the PM adapter 86 for receiving theconnector plug. FIGS. 14 and 15 are, respectively, a plan view and anelevation view of the adapter, and FIG. 16 is a front elevation viewwith connector inserted. The adapter 86 is depicted in the drawings as aduplex adapter, which is a common LC adapter form, but it is to beunderstood that the principles herein set forth may readily be used in asimplex or multiplex adapter.

PM adapter 86 is basically similar to the conventional LC adapter andcomprises a housing 87 made up of first and second plug receivingmembers 88 and 89, each of which has a pair of openings 91 and 92 forreceiving the connector plugs. Each opening has a transverse wall 93 and94 from which project tubular ferrule receiving members 96 and 97 intowhich alignment sleeves 98 and 99 fit. Member 89 is constructed in thesame way so that the alignment sleeves 98 and 99 are situated in theferrule receiving members 96 and 97 in both members 88 and 89 so thatthe members 96 and 97 are aligned. As thus far described, housing 87 isthe same as a conventional LC adapter housing. In order that PMconnections may be realized, each member 88 and 89, which are preferablymade of molded plastic, has spring biasing members 101 and 102 molded inthe outer walls of openings 91 and 92, each has a pad 103 (only one ofwhich is shown) which, when a plug is inserted into either opening 91 or92 bears against the body thereof to produce a positive, repeatable,transverse seating of the plug. Where a simplex adapter is used, thebiasing members 101 and 102 will preferably be located in the side wallsopposition each other.

Further repeatable location of the PM plug is produced by first andsecond slots 104 and 106 for receiving the latching arm 66 of theconnector plug housing 51 which has a truncated wedge shape as discussedin the foregoing. To receive and seat the latching arms, slots 104 and106 have tapered side walls, as best seen in FIGS. 13 and 16, so that,as shown in FIG. 16, latching arms 66 fit snugly therein so thatvirtually any and all transverse float is eliminated.

As shown in FIGS. 13 and 14, PM adapter 86 is configured to be panelmounted. To this end, each of members 88 and 89 has a flange 107 thereonwhich, when the members are assembled together, forms a panel mountingflange. A metallic member 108 straddles member 88 as shown in FIG. 13and is preferably affixed thereto. First and second spring lockingmembers 109 and 111 in the form of cantilevered leaf springs extend frommember 108 as shown in FIGS. 13 and 14 and bear against the back side(or front side) of the panel, shown in dashed lines in FIG. 14, therebylocking adapter 86 in place on the panel.

The adapter as described herein can be used as a conventional LC adapteras well as a PM adapter, and shown and as described herein forms thebasis for U.S. Pat. No. 6,619,856.

Tuning Apparatus and Method

The following discussion is directed to measuring crosstalk in a jumpercable terminated at each end by the connector plug and adapter describedin the foregoing, and to establishing a reference position of theapparatus for tuning the connectors terminating a jumper cable.

As discussed at length in the foregoing, in order to maintainpolarization and optimum light transmission at a connection, it isnecessary to match the slow wave polarization of the jumper connector tothe slow wave polarization of the receiving connector as closely aspossible. FIG. 17 is a diagrammatic representation of the relationshipbetween two ferrules 112 and 113 to be connected together is abuttingrelationship. Contained within ferrule 112 is a PM fiber 114 havingfirst and second stress rods 116 and 117 and which propagates light in aslow axis X and a fast axis Y. Contained within ferrule 113 is a PMfiber 118 having first and second stress rods 119 and 121 andpropagating light in a slow wave X¹ and a fast wave axis Y¹. In orderthat there be optimum light transmission (lowest crosstalk) betweenfibers 114 and 118, slow waves sectors X and X¹ should be parallel or(coincident) and aligned with reference points 122 and 123, which maybe, for example, the latching arms of the connectors in which theferrules 112 and 113 are contained. Unfortunately, it is seldom thatsuch an ideal alignment is obtained. It therefore becomes necessary, foroptimum performance, that the connectors terminating the jumper, forexample, be tuned to at least approach the optimum in performance.

In FIG. 18 there is shown an apparatus 125 for tuning a terminatedjumper cable 126 having PM connectors 127 and 128 as shown and describedin the foregoing. A first assembly 129, to which connector 127 isconnected comprises a light source 131 connected by a connector 130 to acoupling stage 132 having a rotatable polarizer 133 interposed betweenconnectors 130 and 127 through which the light passes to the jumpercable 126. A second assembly 134 spaced from the first assembly 129comprises a coupling stage 136 to which the connector 128 is connected,and a rotatable analyzer 137 through which light is directed to a powermeter 138, the output of which is directed to a processor or computer139. The processor 139 controls a rotation controller 141 for rotatingpolarizer 133 and analyzer 137. With the apparatus set-up 125 as shownin FIG. 18, the crosstalk of jumper cable 126 can be ascertained and theconnectors 127 and 128 can be tuned by the following steps.

Step Ia) Light is launched on fiber 126 from light source 131 throughthe stage 132.

Step IIa) Linear polarized light is then launched into the slow axis ofthe fiber 126.

Step IIIa) Analyzer 137 is rotated as in FIG. 19 to create a transmittedpower graph by means of power meter 138. Such a graph is shown in FIG.20 and gives an indication of transmitted power variation through a full360° of rotation of analyzer 137. It can be seen that the output minimumis more sensitive to angular changes than is the output maximum, whichis 90° of rotation therefrom, thus it is easier to pinpoint the angle atwhich the minimum occurs and simply add 90° to that to determine theangle at which the maximum occurs.

Step IVa) Ascertaining the crosstalk, which is a negative value, bysubtracting the maximum power from the minimum power. In FIG. 20 it canbe seen that the minimum power is −34:2 db at 90° and the maximum poweris −3.4 db at 0°, which yields a crosstalk figure of −30.8 db.

Step Va) It is possible that the output maximum may occur at arotational angle of the analyzer 137 that differs from cable to cable.The PM connector 128 may be tuned by rotating the ferrule, as discussedin the foregoing, and the measurements of steps I-III repeated. Thecrosstalk will not be changed thereby, but the angular position of themaxima and the minima, as indicated by the analyzer 137, will be. Theconnector can, therefore, be incrementally tuned to a setting such as 0°for the maximum, which aligns the slow wave with the connector key, theanalyzer and the software of processor 139 having previously been setfor a zero indication to correspond to connector key alignment with theslow wave.

FIG. 21 is a graph showing variations in crosstalk with misalignment,expressed in degrees of angle. The typical crosstalk of the fiber isapproximately −40 dB per meter, so the characterization of a one meterjumper with no added crosstalk from the connectors would fall on the −40dB curve. On the other hand, where the crosstalk of the connectorterminated jump is −30 dB at approximately zero degrees, the crosstalkvariation with angle would fall on the −30 dB curve. With a crosstalk of−20 dB at approximately zero degrees, the variation with angle followsthe −20 dB curve. Misalignment of one degree (1°) can be dramaticallydifferent, depending upon how good the jumper is. The graph thusillustrates the extreme sensitivity to turning in a connectorizedjumper.

In production milieu, it is desirable to establish a master position forthe analyzer so that jumpers may be tuned thereto at a production rate,with the slow axis of each being aligned with the connector key. Thejoined PM jumpers can have the same slow axis alignment according to keyposition to minimize crosstalk resulting from misalignment. Once thepolarization direction of the analyzer is aligned to the connector key,the tuning process can easily be performed by matching the fiber slowaxis to the analyzer direction as indicated by output light power.

The second stage has a keyed receptacle for receiving the keyedconnector plug thus the keys are aligned with each other. The masterreference position will be that position where the analyzer zerocoincides with the key of the receptacle, and hence the connector key.

In FIG. 22 the apparatus 125 is shown by being set up to tune theconnector plugs 147 and 148 in connector adapter 142 of a pair of jumpercables 143 and 144. Cable 143 is terminated by PM connector plugs 146and 147 of the type disclosed hereinbefore, and cable 144 is terminatedby similar PM connector plugs 148 and 149. The steps in tuning theconnectors are similar to preceding crosstalk measurement and tuning,and are as follows, (refer also to FIGS. 24 through 27);

Step Ib) Measure the crosstalk of the connector as in Steps Ia, IIa, andIIIa;

Step IIb) Adjust all of connectors 147 and 148 to the other for lowestcrostalk;

Step IIIb) Remove adapter 142 and jumper 144 and reconnect connectorplug 147 second coupling stage 136;

Step IVb) Measure the crosstalk of jumper 143 to determine the angularposition of output maximum and minimum, for example, +6° for themaximum; as shown in FIG. 23;

Step Vb) Replace jumper 143 with jumper 144 and reconnect connector plug148 to second stage 136;

Step VIb) Measure the crosstalk of jumper 144 to determine the angularposition of the output maximum and minimum, for example 340° or −20°, asshown in FIG. 23;

Step VIIb) The key position will be at one-half the difference betweenthe two angles, or −7°;

Step VIIIb) Adjust the analyzer, as discussed in the foregoing to −7° asindicated by the analyzer, which aligns the zero angle of the analyzerwith the receptacle key. This is the master reference position. Thesequence or order of jumpers 143 and 144 may be reversed, if desired ornecessary. The connectors can now be tuned for optimum performance. Inthe subsequent production of connectorized fibers, particularly jumpers,it becomes a simple matter to tune the connectors by rotation of theferrule to the key position, with the master reference position set asin the foregoing, thus eliminating the many steps involved in tuning PMconnectors (or jumpers).

In practice, the foregoing method has yielded crosstalk negative valuesof better than −38 db. Because of the unique configuration of the PMconnector plug and the PM connector adapter, when the connector plugsare tuned in accordance with the foregoing steps, the tuning ismaintained in normal usage, due to the reduced float within theconnectors described in the foregoing and resistance to any accidentalor unintentional change of the setting of the ferrule in the connectorplug.

It is to be understood that the various features of the presentinvention lend themselves to other types of PM optical fiber connectors,and that other modifications or adaptations might occur to workers inthe art. All such variations and modifications are intended to beincluded herein as being within the scope of the present invention asset forth. Further, in the claims hereinafter, the correspondingstructures, materials, acts, and equivalents of all means orstep-plus-function elements are intended to include any structure,material or acts for performing the functions in combination with otherelements as specifically claimed.

What is claimed is:
 1. A method of establishing a master reference pointin an apparatus for tuning connectorized PM optical fibers, theapparatus comprising a test set up having a first stage having apolarized light source, at least two PM fibers, a second stage having ananalyzer and a power meter capable of measuring crosstalk and fiber axisdirection, said method comprising: connecting one end of a firstconnectorized PM fiber to the first stage; connecting one end of asecond connectorized PM fiber to the second stage; connecting or matingthe other ends of the first and second fibers to each other by means ofmating connectors; passing light through the fibers from the first stageto the second stage; adjusting the mating connectors of the first andsecond fibers to reduce the crosstalk to the lowest value therein;removing said second fiber and connecting said first fiber to saidsecond stage; passing light through said first fiber towards theanalyzer to the power meter; measuring a first rotational angle of theanalyzer after it is aligned with the first fiber axis; removing saidfirst fiber and connecting said second fiber between the first andsecond stages, where the mating connector of the second fiber isconnected towards analyzer; passing light through said second fibertowards the analyzer to the power meter; measuring a second rotationalangle of the analyzer after it is aligned with the fiber axis;determining the midpoint between the first rotational angle and thesecond rotational angle, which corresponds to a 0° key position;redefining the midpoint as the master reference position of the analyzerfor the test setup.
 2. The method as claimed in claim 1 wherein the stepof connecting said fibers between the first and second stages involvesreversing the order of the first and second fibers to determine therotational angle of the fiber axes.
 3. The method as claimed in claim 1wherein the light passed through the fibers is laser light.
 4. Themethod as claimed in claim 1 and further including the step of applyingthe output of the power meter to a processing unit.
 5. The method asclaimed in claim 4 and further including the step of controlling therotation of the analyzer by means of the processing unit.
 6. The methodas claimed in claim 4 wherein the first stage includes a polarizer andfurther including the step of controlling the rotation of the polarizerby means of the processing unit.